High boiling components of crude oil are unsuitable for inclusion in gasoline and other liquid hydrocarbon fuels. Accordingly, the petroleum refining industry has developed processes for cracking or breaking these high molecular weight, high boiling components into smaller, lower boiling molecules. One cracking process widely used for this purpose is known as fluid catalytic cracking (FCC). The FCC process has reached a highly advanced state, and many variations have been developed, but the unifying characteristic of these variations is that a vaporized hydrocarbon feedstock is cracked by contacting it at an elevated temperature with a cracking catalyst. Upon attainment of the desired degree of molecular weight and boiling point reduction, the catalyst is separated from the desired products.
If the catalyst is reused again and again for processing additional feedstock, which is usually the case, coke and heavy metals deposit onto the catalyst.
The spent catalyst is typically regenerated by contacting it with an oxygen-containing gas under conditions whereby at least a portion of the coke is converted to carbon oxides, and the regenerated catalyst is recycled to the reactor for contact with fresh feed.
As to the heavy metals that accumulate on the catalyst, they eventually accumulate to the point that they unfavorably alter the composition of the catalyst and/or the nature of its effect upon the feedstock. For example, such metals cause an increased formation of coke and hydrogen gas, thereby decreasing the yield of the desired gasoline. In addition, these metals affect both the activity and selectivity of the cracking catalyst. Regeneration does not solve the problems caused by these contaminating metals. Heavy metals capable of adversely affecting the catalytic cracking process include platinum, palladium, chromium, nickel, copper, cobalt, vanadium, and iron. Unfortunately, nickel, copper, vanadium, cobalt, and iron are often present as contaminants in the hydrocarbon feedstocks which are catalytically cracked.
Additional information regarding catalytic cracking of hydrocarbons and its challenges can be found, for example, in U.S. Pat. Nos. 4,454,025 and 4,363,720, which are incorporated by reference.
The ability of various metals and metal compounds to act as metal passivation agents against the adverse effects of transition elements such as nickel, vanadium, cobalt, copper, iron and other heavy metal contaminants on zeolite containing cracking catalysts is known in the art. Such passivating agents are used to enhance or restore metal contaminated, zeolite cracking catalysts. The treatment of the zeolite cracking catalysts with such metal passivation agents provides numerous benefits in catalytic cracking, including higher oil feed conversion, higher gasoline yield, higher isobutylene yield, lower yield of undesirable heavy cycle oil, lower coke generation and/or lower hydrogen gas generation.
Commercially used metal passivation agents come in many forms, including solutions of organometallic complexes and aqueous suspensions of colloidal solid particles in a suspending agent. In aqueous suspensions, the solid particles are typically prepared by chemical precipitation or by ion exchange chemistry. See, for example, U.S. Pat. No. 4,933,095 to Johnson et al., the disclosure of which is incorporated by reference. These passivation agents are used by directly introducing them at a carefully controlled rate into the hydrocarbon catalytic cracking unit, which typically includes a cracking reactor and a catalyst regenerator. For example, they can be introduced into the catalytic cracker, into the hydrocarbon feedstream, or into the regeneration zone. Successful introduction requires that the dispersion of passivating agents be stable and that a reasonable viscosity be maintained.
However, the above passivating agents are often expensive to prepare and the preparation routes are often restricted in the passivating agents that can be made available. In addition, more conventionally available and lower cost solid metals and metal compounds having potential use as effective metal passivation agents are too large to be conveniently suspended to form stable suspensions. Finally, the suspension agents currently used in connection with particulate passivating agents tend to act by thickening the suspension and hence slowing the rate at which the particles settle out of suspension. The use of such suspending agents results in suspensions that have a relatively short shelf life and/or are viscous, making them more difficult and costly to pump.
In view of the above, there is presently a need for a process by which a wide variety of particulate metals and metal compounds useful as metal passivating agents can be provided in a stabilized form that is convenient for introduction into the catalytic cracking process.